1. Field of the Invention
The present invention relates to a PWM modulation circuit using staircase waves for use in mobile equipment or the like and a class D amplifier using such a PWM modulation circuit.
2. Description of the Related Art
In recent years, there has been a demand for compact and efficient amplifiers, and amplifiers having so-called class D amplifiers have widely been used.
FIG. 11 is a block diagram of the configuration of a conventional class D amplifier. The conventional class D amplifier includes an analog signal input terminal 101, a rectangular wave input terminal 102, a PWM modulation circuit 103, an output stage amplifier 104, an L/C filter 105 including an inductor L108 and a capacitor C109, a speaker 106, and a modulated wave output terminal 107.
In the class D amplifier, the PWM modulation circuit 103 modulates an analog signal input from the analog signal input terminal 101 with a rectangular signal input from the rectangular wave input terminal 102 for conversion into a digital signal. The output stage amplifier 104 then amplifies the modulation signal modulated by the PWM modulation circuit 103 and outputs the result to the L/C filter 105 connected to the output stage. The L/C filter 105 and the speaker 106 then return the modulation wave signals amplified by the output state amplifier 104 into an analog signal based on the LPF characteristic. This is how amplification is carried out.
The conventional class D amplifier having the above-described configuration can amplify a digital signal based on the amplitude of the digital signal, in other words, based only on the ON/OFF states of the signal, so that high power efficiency can be achieved.
Another disclosed example of the conventional class D amplifier employs a ternary scheme for differential output voltage signals and the differential outputs in a silent state are in phase, so that no current is passed to the speaker output. In this way, an amplifier removed of an L/C filter is provided (see for example U.S. Pat. No. 6,614,297).
However, the conventional class D amplifier as shown in FIG. 11 uses a rectangular wave for generating a PWM modulated wave, and therefore the ratio of the high frequency component is large as can be seen from the frequency component characteristic of the rectangular wave in the conventional example in FIG. 12. The constant of the L/C filter 105 must be large in order to remove the high frequency component, which can increase the area necessary for packaging the class D amplifier.
According to the disclosure of U.S. Pat. No. 6,614,297, differential outputs are necessary and therefore an additional output terminal for the purpose is necessary. This can also give rise to unwanted increase in the packaging area of the class D amplifier. Furthermore, the disclosed technique cannot be applied with a single-phase output.